1. Field of the Invention
The present invention relates to a numerically controlled or NC system, such as an NC machining system, an NC robot system, or an NC carrier system, and a backlash compensation device for use with such an NC system. More particularly, it relates to improvements for compensating for backlash error produced upon a reversal of the direction of driving an object to be controlled.
2. Description of the Prior Art
For example, prior art NC machining systems can machine an object to be machined (or work) into a complicated contour by securing the object to be machined onto a table and numerically controlling the relative distance between the table and a machine tool. In general, NC machining systems provide three servomotors that independently operate on X, Y, and Z axes perpendicular to each other, respectively, and transmit rotational driving forces generated by the three servomotors to members for supporting the table and the machine tool so as to control the relative distance between the table and the machine tool.
In prior art NC systems, typified by such NC machining systems, there can cause a mismatch between desired amounts of relative travel of the table and work, which are given as control values, and the actual amounts of relative travel of the table and work, due to backlash and play in driving force transmitting mechanisms, such as servomotors disposed as driving sources, ballscrews each for transforming rotary movement of each of the servomotors into a linear movement of a table, coupling between each servomotor and each ballscrew, linear guides for defining the direction in which the table moves, and the table to which either an object to be machined (or work) or a machine tool is secured, when the direction of rotation of each servomotor is reversed. As a result, the contouring accuracy cannot be better than the backlash error.
Referring next to FIG. 14, there is illustrated a block diagram showing the structure of a prior art numerically controlled system, as disclosed in Japanese Patent Application Publication (TOKKAIHEI) 10-154007, which is so constructed as to compensate for backlash error. In the figure, reference numeral 4 denotes a motor, numeral 35 denotes a driving force transmitting unit, and numeral 1 denotes an object to be controlled.
Reference numeral 7 denotes a position command generating unit for generating a position command signal, numeral 8 denotes a command reversal determination unit for determining whether the position command signal has just reversed an upward or downward trend in its amplitude or level, that is, whether or not the direction of driving the object 1 has just been reversed, numeral 36 denotes a displacement counter for obtaining a displacement of the object to be controlled since the reversal of the position command signal, numeral 37 denotes a setting unit for setting a maximum value of backlash compensation, numeral 38 denotes a displacement-dependent compensation computation unit for computing a compensation value corresponding to the displacement that the object to be controlled has undergone since the reversal of the position command signal, numeral 39 denotes a compensation differential value computation unit for computing a compensation differential value based on the compensation value from the displacement-dependent compensation computation unit 38 at predetermined intervals for compensation, numeral 40 denotes an adder for adding the compensation differential value to the position command signal, and numeral 41 denotes a control device for supplying a control current whose magnitude depends on the output of the adder 40 to the motor 4.
In operation, every time the position command signal from the position command generating unit 7 reverses an upward or downward trend in its amplitude or level, the command reversal determination unit 8 furnishes reversal information indicating the fact and the displacement counter 36 obtains a displacement that the object to be controlled has undergone since the reversal of the position command signal. Based on the maximum value for backlash compensation from the setting unit 37 and the displacement of the object to be controlled from the displacement counter 36, the displacement-dependent compensation computation unit 38 generates a backlash compensation value corresponding to the displacement that the object to be controlled has undergone since the reversal of the position command signal. The compensation differential value computation unit 39 then computes a compensation differential value from the backlash compensation value at predetermined intervals for compensation, and the adder 40 adds the compensation differential value to the position command signal from the position command generating unit 7. The control device 41 supplies a control current whose magnitude depends on the output of the adder 40 to the servomotor 4, thus controlling the position of the driving force transmitting unit 35 and hence the position of the object 1 to be controlled.
As previously mentioned, prior art numerically controlled systems make it possible to numerically control an object to be controlled so that it further travels a distance corresponding to the backlash error, every time the motor changes a direction of rotation thereof, by adding a backlash compensation value with respect to the reversed direction of rotation of the motor to the position command value.
Since prior art numerically controlled systems gradually increase the backlash compensation value according to the displacement that the object to be controlled has undergone since the reversal of the direction of rotation of the motor in addition to adding the backlash compensation value to the position command value, they can compensate for the backlash error having a tendency to gradually increase since the reversal of the direction of rotation of the motor.
A problem with prior art numerically controlled systems such as NC machining systems is that a high degree of contouring accuracy cannot be achieved. For example, even though the relative feed velocity of the table against the machine tool is reduced to a very low one in order to provide a high degree of contouring accuracy which cannot be achieved with a normal machining velocity, the above-mentioned prior art compensation method based on the displacement of the object to be controlled cannot compensate for the backlash error effectively because the way that the backlash error occurs is changed when the relative feed velocity of the table against the machine tool is very low. Thus, prior art numerically controlled machining systems cannot achieve a desired high degree of contouring accuracy even though the relative feed velocity of the table against the machine tool is reduced to a very low one in order to improve the contouring accuracy. Especially, the problem about the contouring accuracy, which is caused by backlash, can easily arise when machining an object into a perfect circle with multi axis synchronous control.
In prior art numerically controlled systems, the backlash compensation value can be computed at predetermined intervals for compensation in order to improve the contouring accuracy. An exponential computation that needs many arithmetic operations is needed to compute the backlash compensation value. A certain time interval is thus required for every computation of the backlash compensation value at predetermined intervals, and therefore the number of times that the backlash compensation value is updated per time is limited. As a result, the time period during which the backlash error is compensated for with the current backlash compensation value must be increased, and an error introduced into the backlash compensation value with respect to the backlash error can be increased. Thus a desired high degree of contouring accuracy cannot be provided.
Instead of the above-mentioned semi-closed feedback method, a full-closed feedback method comprising the steps of detecting the position of the table, and compensating for the amount of rotation of the motor based on the detected position of the table can be applied to prior art numerically controlled systems in order to reduce backlash error. The full-closed feedback method is not, however, suitable for practical use because in addition to that the number of components required for the full-closed feedback method is increased, the accuracy of positioning each component must be improved, a response delay in the feedback system can introduce an error in the contouring accuracy, and the maximum magnitude of the feed velocity can be reduced.
The inventor has devoted himself to study to solve the above problems and to provide a high degree of contouring accuracy, and has found that while any change in the feed velocity can greatly vary a correlation between a tendency to gradually increase after a reversal of the direction of rotation of each servomotor that the backlash error shows, and the displacement that the object to be controlled has undergone since the reversal of the direction of rotation of each servomotor, any change in the feed velocity can hardly vary a correlation between a tendency to gradually increase after a reversal of the direction of rotation of each servomotor that the backlash error shows, and how much time has elapsed since the reversal of the direction of rotation of each servomotor, that is, the way that the backlash error increases after the reversal of the direction of rotation of each servomotor can hardly vary with respect to the time elapsed since the reversal of the direction of rotation of each servomotor even though the feed velocity is changed. The inventor further found that even in the case of multi axis synchronous control that makes changes in the feed velocity, a correlation between a tendency to gradually increase after a reversal of the direction of rotation of each servomotor that the backlash error shows, and how much time has elapsed since the reversal of the direction of rotation of each servomotor can hardly vary regardless of changes in the feed velocity. The inventor has completed the present invention based on the found correlation.